Method and apparatus for detecting arrhythmias in a medical device

ABSTRACT

A method of detecting a cardiac event in a medical device that includes determining a first characteristic in response to cardiac signals sensed along a first sensing vector over a predetermined sensing window and in response to cardiac signals sensed along a second sensing vector over the predetermined sensing window, determining a second characteristic in response to cardiac signals sensed along the first sensing vector over the predetermined sensing window and in response to cardiac signals sensed along the second sensing vector over the predetermined sensing window, and determining a third characteristic in response to cardiac signals sensed along the first sensing vector over the predetermined sensing window and in response to cardiac signals sensed along the second sensing vector over the predetermined sensing window.

RELATED APPLICATION

The present application claims priority and other benefits from U.S.Provisional Patent Application Ser. No. 60/786,981, filed Mar. 29, 2006,entitled “METHOD AND APPARATUS FOR DETECTING ARRHYTHMIAS IN ASUBCUTANEOUS MEDICAL DEVICE”, incorporated herein by reference in itsentirety.

CROSS-REFERENCE TO RELATED APPLICATIONS

Cross-reference is hereby made to the commonly assigned related U.S.applications, U.S. patent application Ser. No. 11/380,794, entitled“METHOD AND APPARATUS FOR DETECTING ARRHYTHMIAS IN A MEDICAL DEVICE”, toGhanem et al.; U.S. patent application Ser. No. 11/380,823, entitled“METHOD AND APPARATUS FOR DETECTING ARRHYTHMIAS IN A MEDICAL DEVICE”, toGhanem et al.; U.S. patent application Ser. No. 11/380,815, entitled“METHOD AND APPARATUS FOR DETECTING ARRHYTHMIAS IN A MEDICAL DEVICE”, toGhanem et al.; U.S. patent application Ser. No. 11/380,842, entitled“METHOD AND APPARATUS FOR DETECTING ARRHYTHMIAS IN A MEDICAL DEVICE”, toGhanem et al.; U.S. patent application Ser. No. 11/380,823, entitled“METHOD AND APPARATUS FOR DETECTING ARRHYTHMIAS IN A MEDICAL DEVICE”, toGhanem et al.; U.S. patent application Ser. No. 11/380,807, entitled“METHOD AND APPARATUS FOR DETECTING ARRHYTHMIAS IN A MEDICAL DEVICE”, toGhanem et al.; U.S. patent application Ser. No. 11/380,837, entitled“METHOD AND APPARATUS FOR DETECTING ARRHYTHMIAS IN A MEDICAL DEVICE”, toGhanem et al.; and U.S. patent application Ser. No. 11/380,802, entitled“METHOD AND APPARATUS FOR DETECTING ARRHYTHMIAS IN A MEDICAL DEVICE”, toGhanem et al.; all filed concurrently herewith and incorporated hereinby reference in their entirety.

FIELD OF THE INVENTION

The present invention generally relates to an implantable medical devicesystem, and more particularly to a method and apparatus for detectingarrhythmias in a subcutaneous medical device.

BACKGROUND OF THE INVENTION

Many types of implantable medical devices (IMDs) have been implantedthat deliver relatively high-energy cardioversion and/or defibrillationshocks to a patient's heart when a malignant tachyarrhythmia, e.g.,ventricular tachycardia or ventricular fibrillation, is detected.Cardioversion shocks are typically delivered in synchrony with adetected R-wave when fibrillation detection criteria are met, whereasdefibrillation shocks are typically delivered when fibrillation criteriaare met and an R-wave cannot be discerned from the electrogram (EGM).

The current state of the art of ICDs or implantablepacemaker/cardioverter/defibrillators (PCDs) includes a full featuredset of extensive programmable parameters which includes multiplearrhythmia detection criteria, multiple therapy prescriptions (forexample, stimulation for pacing in the atrial, ventricular and/or bothchambers, bi-atrial and/or bi-ventricular pacing, arrhythmia overdriveor entrainment stimulation, and high level stimulation for cardioversionand/or defibrillation), extensive diagnostic capabilities and high speedtelemetry systems.

Current technology for the implantation of an IMD uses a transvenousapproach for cardiac electrodes and lead wires. The defibrillatorcanister/housing is generally implanted as an active can fordefibrillation and electrodes positioned in the heart are used forpacing, sensing and detection of arrhythmias.

Attempts are being made to identify patients who are asymptomatic byconventional measures but are nevertheless at risk of a future suddendeath episode. Current studies of patient populations, e.g., the MADITII and SCDHeFT studies, are establishing that there are large numbers ofpatients in any given population that are susceptible to sudden cardiacdeath, that they can be identified with some degree of certainty andthat they are candidates for a prophylactic implantation of adefibrillator (often called primary prevention).

One option proposed for this patient population is to implant aprophylactic subcutaneous implantable device (SubQ device). As SubQdevice technology evolves, it may develop a clear and distinct advantageover non-SubQ devices. For example, the SubQ device does not requireleads to be placed in the bloodstream. Accordingly, complicationsarising from leads placed in the cardiovasculature environment areeliminated. Further, endocardial lead placement is not possible withpatients who have a mechanical heart valve implant and is not generallyrecommended for pediatric cardiac patients. For these and other reasons,a SubQ device may be preferred over an ICD.

There are technical challenges associated with the operation of a SubQdevice. For example, SubQ device sensing is challenged by the presenceof muscle artifact, respiration and other physiological signal sources.This is particularly because the SubQ device is limited to far-fieldsensing since there are no intracardial or epicardial electrodes in asubcutaneous system. Further, sensing of atrial activation fromsubcutaneous electrodes is limited since the atria represent a smallmuscle mass and the atrial signals are not sufficiently detectabletransthoracically.

Yet another challenge could occur in situations where it is desirable tocombine a SubQ device with an existing pacemaker (IPG) in a patient.While this may be desirable in a case where an IPG patient may need adefibrillator, a combination implant of a SubQ device and an IPG mayresult in inappropriate therapy by the SubQ device, which may pace orshock based on spikes from the IPG. Specifically, each time the IPGemits a pacing stimulus, the SubQ device may interpret it as a genuinecardiac beat. The result can be over-counting beats from the atrium,ventricles or both; or, because of the larger pacing spikes, sensing ofarrhythmic signals (which are typically much smaller in amplitude) maybe compromised.

Therefore, for these and other reasons, a need exists for an improvedmethod and apparatus to reliably sense and detect arrhythmias in asubcutaneous device, while rejecting noise and other physiologicsignals.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects and features of the present invention will be appreciated as thesame becomes better understood by reference to the following detaileddescription of the embodiments of the invention when considered inconnection with the accompanying drawings, wherein:

FIG. 1 is a schematic diagram of an exemplary subcutaneous device inwhich the present invention may be usefully practiced;

FIG. 2 is a schematic diagram of the implantable medical device of FIG.1, according to one exemplary embodiment of the invention.

FIG. 3 is an exemplary schematic diagram of electronic circuitry withina hermetically sealed housing of a subcutaneous device of the presentinvention;

FIG. 4 is a schematic diagram of signal processing aspects of asubcutaneous device according to an exemplary embodiment of the presentinvention;

FIG. 5 is a state diagram of detection of arrhythmias in a subcutaneousdevice according to an embodiment of the present invention;

FIG. 6 is a flow chart of a method for detecting arrhythmias in asubcutaneous device according to an embodiment of the present invention;

FIGS. 7A-7I are flow charts of a method for detecting arrhythmias in asubcutaneous device according to an embodiment of the present invention;

FIG. 8 is a graphical representation of sensing of cardiac activityaccording to an embodiment of the present invention;

FIG. 9A is a graphical representation of a determination of whether asignal is corrupted by muscle noise according to an embodiment of thepresent invention;

FIG. 9B is a flowchart of a method of determining whether a signal iscorrupted by muscle noise according to an embodiment of the presentinvention;

FIG. 9C is a flowchart of a method of determining whether a signal iscorrupted by muscle noise according to an embodiment of the presentinvention;

FIG. 10 is a graphical representation of a VF shock zone according to anembodiment of the present invention; and

FIGS. 11A and 11B are graphical representations of the determination ofwhether an event is within a shock zone according to an embodiment ofthe present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a schematic diagram of an exemplary subcutaneous device inwhich the present invention may be usefully practiced. As illustrated inFIG. 1, a subcutaneous device 14 according to an embodiment of thepresent invention is subcutaneously implanted outside the ribcage of apatient 12, anterior to the cardiac notch. Further, a subcutaneoussensing and cardioversion/defibrillation therapy delivery lead 18 inelectrical communication with subcutaneous device 14 is tunneledsubcutaneously into a location adjacent to a portion of a latissimusdorsi muscle of patient 12. Specifically, lead 18 is tunneledsubcutaneously from the median implant pocket of the subcutaneous device14 laterally and posterially to the patient's back to a locationopposite the heart such that the heart 16 is disposed between thesubcutaneous device 14 and the distal electrode coil 24 and distalsensing electrode 26 of lead 18.

It is understood that while the subcutaneous device 14 is shownpositioned through loose connective tissue between the skin and musclelayer of the patient, the term “subcutaneous device” is intended toinclude a device that can be positioned in the patient to be implantedusing any non-intravenous location of the patient, such as below themuscle layer or within the thoracic cavity, for example.

Further referring to FIG. 1, a programmer 20 is shown in telemetriccommunication with subcutaneous device 14 by an RF communication link22. Communication link 22 may be any appropriate RF link such asBluetooth, WiFi, MICS, or as described in U.S. Pat. No. 5,683,432“Adaptive Performance-Optimizing Communication System for Communicatingwith an Implantable Medical Device” to Goedeke, et al and incorporatedherein by reference in its entirety.

Subcutaneous device 14 includes a housing 15 that may be constructed ofstainless steel, titanium or ceramic as described in U.S. Pat. No.4,180,078 “Lead Connector for a Body Implantable Stimulator” to Andersonand U.S. Pat. No. 5,470,345 “Implantable Medical Device withMulti-layered Ceramic Enclosure” to Hassler, et al, both incorporatedherein by reference in their entireties. The electronics circuitry ofSubQ ICD 14 may be incorporated on a polyimide flex circuit, printedcircuit board (PCB) or ceramic substrate with integrated circuitspackaged in leadless chip carriers and/or chip scale packaging (CSP).

Subcutaneous lead 18 includes a distal defibrillation coil electrode 24,a distal sensing electrode 26, an insulated flexible lead body and aproximal connector pin 27 (shown in FIG. 2) for connection to thehousing 15 of the subcutaneous device 14 via a connector 25. Inaddition, one or more electrodes 28 (shown in FIG. 2) are positionedalong the outer surface of the housing to form a housing-basedsubcutaneous electrode array (SEA). Distal sensing electrode 26 is sizedappropriately to match the sensing impedance of the housing-basedsubcutaneous electrode array.

It is understood that while device 14 is shown with electrodes 28positioned on housing 15, according to an embodiment of the presentinvention electrodes 28 may be alternatively positioned along one ormore separate leads connected to device 14 via connector 25.

Continuing with FIG. 2, electrodes 28 are welded into place on theflattened periphery of the housing 15. In the embodiment depicted inthis figure, the complete periphery of the SubQ ICD may be manufacturedto have a slightly flattened perspective with rounded edges toaccommodate the placement of the electrodes 28. The electrodes 28 arewelded to housing 15 (to preserve hermaticity) and are connected viawires (not shown) to electronic circuitry (described herein below)inside housing 15. Electrodes 28 may be constructed of flat plates, oralternatively, may be spiral electrodes as described in U.S. Pat. No.6,512,940 “Subcutaneous Spiral Electrode for Sensing Electrical Signalsof the Heart” to Brabec, et al and mounted in a non-conductive surroundshroud as described in U.S. Pat. No. 6,522,915 “Surround ShroudConnector and Electrode Housings for a Subcutaneous Electrode Array andLeadless ECGs” to Ceballos, et al and U.S. Pat. No. 6,622,046“Subcutaneous Sensing Feedthrough/Electrode Assembly” to Fraley, et al,all incorporated herein by reference in their entireties. The electrodes28 of FIG. 2 can be positioned to form orthogonal or equilateral signalvectors, for example.

The electronic circuitry employed in subcutaneous device 14 can take anyof the known forms that detect a tachyarrhythmia from the sensed ECG andprovide cardioversion/defibrillation shocks as well as post-shock pacingas needed while the heart recovers. A simplified block diagram of suchcircuitry adapted to function employing the first and secondcardioversion-defibrillation electrodes as well as the ECG sensing andpacing electrodes described herein below is set forth in FIG. 3. It willbe understood that the simplified block diagram does not show all of theconventional components and circuitry of such devices including digitalclocks and clock lines, low voltage power supply and supply lines forpowering the circuits and providing pacing pulses or telemetry circuitsfor telemetry transmissions between the device 14 and externalprogrammer 20.

FIG. 3 is an exemplary schematic diagram of electronic circuitry withina hermetically sealed housing of a subcutaneous device according to anembodiment of the present invention. As illustrated in FIG. 3,subcutaneous device 14 includes a low voltage battery 153 coupled to apower supply (not shown) that supplies power to the circuitry of thesubcutaneous device 14 and the pacing output capacitors to supply pacingenergy in a manner well known in the art. The low voltage battery 153may be formed of one or two conventional LiCF_(x), LiMnO₂ or Lil₂ cells,for example. The subcutaneous device 14 also includes a high voltagebattery 112 that may be formed of one or two conventional LiSVO orLiMnO₂ cells. Although both a low voltage battery and a high voltagebattery are shown in FIG. 3, according to an embodiment of the presentinvention, the device 14 could utilize a single battery for both highand low voltage uses.

Further referring to FIG. 3, subcutaneous device 14 functions arecontrolled by means of software, firmware and hardware thatcooperatively monitor the ECG, determine when acardioversion-defibrillation shock or pacing is necessary, and deliverprescribed cardioversion-defibrillation and pacing therapies. Thesubcutaneous device 14 may incorporate circuitry set forth in commonlyassigned U.S. Pat. No. 5,163,427 “Apparatus for Delivering Single andMultiple Cardioversion and Defibrillation Pulses” to Keimel and U.S.Pat. No. 5,188,105 “Apparatus and Method for Treating a Tachyarrhythmia”to Keimel for selectively delivering single phase, simultaneous biphasicand sequential biphasic cardioversion-defibrillation shocks typicallyemploying ICD IPG housing electrodes 28 coupled to the COMMON output 123of high voltage output circuit 140 and cardioversion-defibrillationelectrode 24 disposed posterially and subcutaneously and coupled to theHVI output 113 of the high voltage output circuit 140. Outputs 132 ofFIG. 3 is coupled to sense electrode 26.

The cardioversion-defibrillation shock energy and capacitor chargevoltages can be intermediate to those supplied by ICDs having at leastone cardioversion-defibrillation electrode in contact with the heart andmost AEDs having cardioversion-defibrillation electrodes in contact withthe skin. The typical maximum voltage necessary for ICDs using mostbiphasic waveforms is approximately 750 Volts with an associated maximumenergy of approximately 40 Joules. The typical maximum voltage necessaryfor AEDs is approximately 2000-5000 Volts with an associated maximumenergy of approximately 200-360 Joules depending upon the model andwaveform used. The subcutaneous device 14 of the present invention usesmaximum voltages in the range of about 300 to approximately 1000 Voltsand is associated with energies of approximately 25 to 150 joules ormore. The total high voltage capacitance could range from about 50 toabout 300 microfarads. Such cardioversion-defibrillation shocks are onlydelivered when a malignant tachyarrhythmia, e.g., ventricularfibrillation is detected through processing of the far field cardiac ECGemploying the detection algorithms as described herein below.

In FIG. 3, sense amp 190 in conjunction with pacer/device timing circuit178 processes the far field ECG sense signal that is developed across aparticular ECG sense vector defined by a selected pair of thesubcutaneous electrodes 24, 26 and 28, or, optionally, a virtual signal(i.e., a mathematical combination of two vectors) if selected. Theselection of the sensing electrode pair is made through the switchmatrix/MUX 191 in a manner to provide the most reliable sensing of theECG signal of interest, which would be the R wave for patients who arebelieved to be at risk of ventricular fibrillation leading to suddendeath. The far field ECG signals are passed through the switchmatrix/MUX 191 to the input of the sense amplifier 190 that, inconjunction with pacer/device timing circuit 178, evaluates the sensedEGM. Bradycardia, or asystole, is typically determined by an escapeinterval timer within the pacer timing circuit 178 and/or the controlcircuit 144. Pace Trigger signals are applied to the pacing pulsegenerator 192 generating pacing stimulation when the interval betweensuccessive R-waves exceeds the escape interval. Bradycardia pacing isoften temporarily provided to maintain cardiac output after delivery ofa cardioversion-defibrillation shock that may cause the heart to slowlybeat as it recovers back to normal function. Sensing subcutaneous farfield signals in the presence of noise may be aided by the use ofappropriate denial and extensible accommodation periods as described inU.S. Pat. No. 6,236,882 “Noise Rejection for Monitoring ECGs” to Lee, etal and incorporated herein by reference in its' entirety.

Detection of a malignant tachyarrhythmia is determined in the Controlcircuit 144 as a function of the intervals between R-wave sense eventsignals that are output from the pacer/device timing 178 and senseamplifier circuit 190 to the timing and control circuit 144. It shouldbe noted that the present invention utilizes not only interval basedsignal analysis method but also supplemental sensors and morphologyprocessing method and apparatus as described herein below.

Supplemental sensors such as tissue color, tissue oxygenation,respiration, patient activity and the like may be used to contribute tothe decision to apply or withhold a defibrillation therapy as describedgenerally in U.S. Pat. No. 5,464,434 “Medical Interventional DeviceResponsive to Sudden Hemodynamic Change” to Alt and incorporated hereinby reference in its entirety. Sensor processing block 194 providessensor data to microprocessor 142 via data bus 146. Specifically,patient activity and/or posture may be determined by the apparatus andmethod as described in U.S. Pat. No. 5,593,431 “Medical ServiceEmploying Multiple DC Accelerometers for Patient Activity and PostureSensing and Method” to Sheldon and incorporated herein by reference inits entirety. Patient respiration may be determined by the apparatus andmethod as described in U.S. Pat. No. 4,567,892 “Implantable CardiacPacemaker” to Plicchi, et al and incorporated herein by reference in itsentirety. Patient tissue oxygenation or tissue color may be determinedby the sensor apparatus and method as described in U.S. Pat. No.5,176,137 to Erickson, et al and incorporated herein by reference in itsentirety. The oxygen sensor of the '137 patent may be located in thesubcutaneous device pocket or, alternatively, located on the lead 18 toenable the sensing of contacting or near-contacting tissue oxygenationor color.

Certain steps in the performance of the detection algorithm criteria arecooperatively performed in microcomputer 142, including microprocessor,RAM and ROM, associated circuitry, and stored detection criteria thatmay be programmed into RAM via a telemetry interface (not shown)conventional in the art. Data and commands are exchanged betweenmicrocomputer 142 and timing and control circuit 144, pacertiming/amplifier circuit 178, and high voltage output circuit 140 via abi-directional data/control bus 146. The pacer timing/amplifier circuit178 and the control circuit 144 are clocked at a slow clock rate. Themicrocomputer 142 is normally asleep, but is awakened and operated by afast clock by interrupts developed by each R-wave sense event, onreceipt of a downlink telemetry programming instruction or upon deliveryof cardiac pacing pulses to perform any necessary mathematicalcalculations, to perform tachycardia and fibrillation detectionprocedures, and to update the time intervals monitored and controlled bythe timers in pacer/device timing circuitry 178.

When a malignant tachycardia is detected, high voltage capacitors 156,158, 160, and 162 are charged to a pre-programmed voltage level by ahigh-voltage charging circuit 164. It is generally consideredinefficient to maintain a constant charge on the high voltage outputcapacitors 156, 158, 160, 162. Instead, charging is initiated whencontrol circuit 144 issues a high voltage charge command HVCHG deliveredon line 145 to high voltage charge circuit 164 and charging iscontrolled by means of bi-directional control/data bus 166 and afeedback signal VCAP from the HV output circuit 140. High voltage outputcapacitors 156, 158, 160 and 162 may be of film, aluminum electrolyticor wet tantalum construction.

The negative terminal of high voltage battery 112 is directly coupled tosystem ground. Switch circuit 114 is normally open so that the positiveterminal of high voltage battery 112 is disconnected from the positivepower input of the high voltage charge circuit 164. The high voltagecharge command HVCHG is also conducted via conductor 149 to the controlinput of switch circuit 114, and switch circuit 114 closes in responseto connect positive high voltage battery voltage EXT B+ to the positivepower input of high voltage charge circuit 164. Switch circuit 114 maybe, for example, a field effect transistor (FET) with itssource-to-drain path interrupting the EXT B+ conductor 118 and its gatereceiving the HVCHG signal on conductor 145. High voltage charge circuit164 is thereby rendered ready to begin charging the high voltage outputcapacitors 156, 158, 160, and 162 with charging current from highvoltage battery 112.

High voltage output capacitors 156, 158, 160, and 162 may be charged tovery high voltages, e.g., 300-1000V, to be discharged through the bodyand heart between the electrode pair of subcutaneouscardioversion-defibrillation electrodes 113 and 123. The details of thevoltage charging circuitry are also not deemed to be critical withregard to practicing the present invention; one high voltage chargingcircuit believed to be suitable for the purposes of the presentinvention is disclosed. High voltage capacitors 156, 158, 160 and 162may be charged, for example, by high voltage charge circuit 164 and ahigh frequency, high-voltage transformer 168 as described in detail incommonly assigned U.S. Pat. No. 4,548,209 “Energy Converter forImplantable Cardioverter” to Wielders, et al. Proper charging polaritiesare maintained by diodes 170, 172, 174 and 176 interconnecting theoutput windings of high-voltage transformer 168 and the capacitors 156,158, 160, and 162. As noted above, the state of capacitor charge ismonitored by circuitry within the high voltage output circuit 140 thatprovides a VCAP, feedback signal indicative of the voltage to the timingand control circuit 144. Timing and control circuit 144 terminates thehigh voltage charge command HVCHG when the VCAP signal matches theprogrammed capacitor output voltage, i.e., thecardioversion-defibrillation peak shock voltage.

Control circuit 144 then develops first and second control signalsNPULSE 1 and NPULSE 2, respectively, that are applied to the highvoltage output circuit 140 for triggering the delivery of cardiovertingor defibrillating shocks. In particular, the NPULSE 1 signal triggersdischarge of the first capacitor bank, comprising capacitors 156 and158. The NPULSE 2 signal triggers discharge of the first capacitor bankand a second capacitor bank, comprising capacitors 160 and 162. It ispossible to select between a plurality of output pulse regimes simply bymodifying the number and time order of assertion of the NPULSE 1 andNPULSE 2 signals. The NPULSE 1 signals and NPULSE 2 signals may beprovided sequentially, simultaneously or individually. In this way,control circuitry 144 serves to control operation of the high voltageoutput stage 140, which delivers high energycardioversion-defibrillation shocks between the pair of thecardioversion-defibrillation electrodes 113 and 123 coupled to the HV-1and COMMON output as shown in FIG. 3.

Thus, subcutaneous device 14 monitors the patient's cardiac status andinitiates the delivery of a cardioversion-defibrillation shock throughthe cardioversion-defibrillation electrodes 24 and 28 in response todetection of a tachyarrhythmia requiring cardioversion-defibrillation.The high HVCHG signal causes the high voltage battery 112 to beconnected through the switch circuit 114 with the high voltage chargecircuit 164 and the charging of output capacitors 156, 158, 160, and 162to commence. Charging continues until the programmed charge voltage isreflected by the VCAP signal, at which point control and timing circuit144 sets the HVCHG signal low terminating charging and opening switchcircuit 114. Typically, the charging cycle takes only fifteen to twentyseconds, and occurs very infrequently. The subcutaneous device 14 can beprogrammed to attempt to deliver cardioversion shocks to the heart inthe manners described above in timed synchrony with a detected R-wave orcan be programmed or fabricated to deliver defibrillation shocks to theheart in the manners described above without attempting to synchronizethe delivery to a detected R-wave. Episode data related to the detectionof the tachyarrhythmia and delivery of the cardioversion-defibrillationshock can be stored in RAM for uplink telemetry transmission to anexternal programmer as is well known in the art to facilitate indiagnosis of the patient's cardiac state. A patient receiving the device14 on a prophylactic basis would be instructed to report each suchepisode to the attending physician for further evaluation of thepatient's condition and assessment for the need for implantation of amore sophisticated ICD.

Subcutaneous device 14 desirably includes telemetry circuit (not shownin FIG. 3), so that it is capable of being programmed by means ofexternal programmer 20 via a 2-way telemetry link 22 (shown in FIG. 1).Uplink telemetry allows device status and diagnostic/event data to besent to external programmer 20 for review by the patient's physician.Downlink telemetry allows the external programmer via physician controlto allow the programming of device function and the optimization of thedetection and therapy for a specific patient. Programmers and telemetrysystems suitable for use in the practice of the present invention havebeen well known for many years. Known programmers typically communicatewith an implanted device via a bi-directional radio-frequency telemetrylink, so that the programmer can transmit control commands andoperational parameter values to be received by the implanted device, sothat the implanted device can communicate diagnostic and operationaldata to the programmer. Programmers believed to be suitable for thepurposes of practicing the present invention include the Models 9790 andCareLink® programmers, commercially available from Medtronic, Inc.,Minneapolis, Minn.

Various telemetry systems for providing the necessary communicationschannels between an external programming unit and an implanted devicehave been developed and are well known in the art. Telemetry systemsbelieved to be suitable for the purposes of practicing the presentinvention are disclosed, for example, in the following U.S. Patents:U.S. Pat. No. 5,127,404 to Wyborny et al. entitled “Telemetry Format forImplanted Medical Device”; U.S. Pat. No. 4,374,382 to Markowitz entitled“Marker Channel Telemetry System for a Medical Device”; and U.S. Pat.No. 4,556,063 to Thompson et al. entitled “Telemetry System for aMedical Device”. The Wyborny et al. '404, Markowitz '382, and Thompsonet al. '063 patents are commonly assigned to the assignee of the presentinvention, and are each hereby incorporated by reference herein in theirrespective entireties.

FIG. 4 is a schematic diagram of signal processing aspects of asubcutaneous device according to an exemplary embodiment of the presentinvention. The transthoracic ECG signal (ECG1) detected between thedistal electrode 26 of subcutaneous lead 18 and one of electrodes 28positioned on the subcutaneous device 14 are amplified and bandpassfiltered (2.5-105 Hz) by pre-amplifiers 202 and 206 located in Sense Amp190 of FIG. 3. The amplified EGM signals are directed to A/D converters210 and 212, which operate to sample the time varying analog EGM signaland digitize the sampled points. The digital output of A/D converters210 and 212 are applied to temporary buffers/control logic, which shiftsthe digital data through its stages in a FIFO manner under the controlof Pacer/Device Timing block 178 of FIG. 3. Virtual Vector block 226selects one housing-based ECG signal (ECG2) from any pair of electrodes28 as described, for example, in U.S. Pat. No. 5,331,966 “SubcutaneousMulti-Electrode Sensing System, Method and Pacer” to Bennett, et al or,alternatively, generates a virtual vector signal under control ofMicroprocessor 142 and Control block 144 as described in U.S. Pat. No.6,505,067 “System and Method for Deriving Virtual ECG or EGM Signal” toLee, et al; both patents incorporated herein by reference in theirentireties. ECG1 and ECG2 vector selection may be selected by thepatient's physician and programmed via telemetry link 22 from programmer20.

According to an embodiment of the present invention, in order toautomatically select the preferred ECG vector set, it is necessary tohave an index of merit upon which to rate the quality of the signal.“Quality” is defined as the signal's ability to provide accurate heartrate estimation and accurate morphological waveform separation betweenthe patient's usual sinus rhythm and the patient's ventriculartachyarrhythmia.

Appropriate indices may include R-wave amplitude, R-wave peak amplitudeto waveform amplitude between R-waves (i.e., signal to noise ratio), lowslope content, relative high versus low frequency power, mean frequencyestimation, probability density function, or some combination of thesemetrics.

Automatic vector selection might be done at implantation or periodically(daily, weekly, monthly) or both. At implant, automatic vector selectionmay be initiated as part of an automatic device turn-on procedure thatperforms such activities as measure lead impedances and batteryvoltages. The device turn-on procedure may be initiated by theimplanting physician (e.g., by pressing a programmer button) or,alternatively, may be initiated automatically upon automatic detectionof device/lead implantation. The turn-on procedure may also use theautomatic vector selection criteria to determine if ECG vector qualityis adequate for the current patient and for the device and leadposition, prior to suturing the subcutaneous device 14 device in placeand closing the incision. Such an ECG quality indicator would allow theimplanting physician to maneuver the device to a new location ororientation to improve the quality of the ECG signals as required. Thepreferred ECG vector or vectors may also be selected at implant as partof the device turn-on procedure. The preferred vectors might be thosevectors with the indices that maximize rate estimation and detectionaccuracy. There may also be an a priori set of vectors that arepreferred by the physician, and as long as those vectors exceed someminimum threshold, or are only slightly worse than some other moredesirable vectors, the a priori preferred vectors are chosen. Certainvectors may be considered nearly identical such that they are not testedunless the a priori selected vector index falls below some predeterminedthreshold.

Depending upon metric power consumption and power requirements of thedevice, the ECG signal quality metric may be measured on the range ofvectors (or alternatively, a subset) as often as desired. Data may begathered, for example, on a minute, hourly, daily, weekly or monthlybasis. More frequent measurements (e.g., every minute) may be averagedover time and used to select vectors based upon susceptibility ofvectors to occasional noise, motion noise, or EMI, for example.

Alternatively, the subcutaneous device 14 may have an indicator/sensorof patient activity (piezo-resistive, accelerometer, impedance, or thelike) and delay automatic vector measurement during periods of moderateor high patient activity to periods of minimal to no activity. Onerepresentative scenario may include testing/evaluating ECG vectors oncedaily or weekly while the patient has been determined to be asleep(using an internal clock (e.g., 2:00 am) or, alternatively, infer sleepby determining the patient's position (via a 2- or 3-axis accelerometer)and a lack of activity).

If infrequent automatic, periodic measurements are made, it may also bedesirable to measure noise (e.g., muscle, motion, EMI, etc.) in thesignal and postpone the vector selection measurement when the noise hassubsided.

Subcutaneous device 14 may optionally have an indicator of the patient'sposture (via a 2- or 3-axis accelerometer). This sensor may be used toensure that the differences in ECG quality are not simply a result ofchanging posture/position. The sensor may be used to gather data in anumber of postures so that ECG quality may be averaged over thesepostures or, alternatively, selected for a preferred posture.

In the preferred embodiment, vector quality metric calculations wouldoccur a number of times over approximately 1 minute, once per day, foreach vector. These values would be averaged for each vector over thecourse of one week. Averaging may consist of a moving average orrecursive average depending on time weighting and memory considerations.In this example, the preferred vector(s) would be selected once perweek.

Continuing with FIG. 4, a diagnostic channel 228 receives a programmableselected ECG signal from the housing based subcutaneous electrodes andthe transthoracic ECG from the distal electrode 26 on lead 18. Block 238compresses the digital data, the data is applied to temporarybuffers/control logic 218 which shifts the digital data through itsstages in a FIFO manner under the control of Pacer/Device Timing block178 of FIG. 3, and the data is then stored in SRAM block 244 via directmemory access block 242.

The two selected ECG signals (ECG1 and ECG2) are additionally used toprovide R-wave interval sensing via ECG sensing block 230. IIR notchfilter block 246 provides 50/60 Hz notch filtering. A rectifier andauto-threshold block 248 provides R-wave event detection as described inU.S. Pat. No. 5,117,824 “Apparatus for Monitoring Electrical PhysiologicSignals” to Keimel, et al; publication WO2004023995 “Method andApparatus for Cardiac R-wave Sensing in a Subcutaneous ECG Waveform” toCao, et al and U.S. Publication No. 2004/0260350 “Automatic EGMAmplitude Measurements During Tachyarrhythmia Episodes” to Brandstetter,et al, all incorporated herein by reference in their entireties. Therectifier of block 248 performs full wave rectification on theamplified, narrowband signal from bandpass filter 246. A programmablefixed threshold (percentage of peak value), a moving average or, morepreferably, an auto-adjusting threshold is generated as described in the'824 patent or '350 publication. In these references, following adetected depolarization, the amplifier is automatically adjusted so thatthe effective sensing threshold is set to be equal to a predeterminedportion of the amplitude of the sensed depolarization, and the effectivesensing threshold decays thereafter to a lower or base-sensingthreshold. A comparator in block 248 determines signal crossings fromthe rectified waveform and auto-adjusting threshold signal. A timerblock 250 provides R-wave to R-wave interval timing for subsequentarrhythmia detection (to be described herein below). The heart rateestimation is derived from the last 12 R-R intervals (e.g., by a mean,trimmed mean, or median, for example), with the oldest data value beingremoved as a new data value is added.

FIG. 5 is a schematic diagram of a rectifier and auto-threshold unit ina subcutaneous device according to an embodiment of the presentinvention. Waveform 402 depicts a typical subcutaneous ECG waveform andwaveform 404 depicts the same waveform after filtering andrectification. A time dependant threshold 406 allows a more sensitivesensing threshold temporally with respect to the previous sensed R-wave.Sensed events 408 indicate when the rectified ECG signal 404 exceeds theauto-adjusting threshold and a sensed event has occurred.

Returning to FIG. 4, the subcutaneous ECG signal (ECG1) is applied toECG morphology block 232, filtered by a 2-pole 23 Hz low pass filter 252and evaluated by DSP microcontroller 254 under control of programinstructions stored in System Instruction RAM 258. ECG morphology isused for subsequent rhythm detection/determination (to be describedherein below).

FIG. 6 is a state diagram of detection of arrhythmias in a medicaldevice according to an embodiment of the present invention. Asillustrated in FIG. 6, during normal operation, the device 14 is in anot concerned state 302, described in more detail herein below, duringwhich R-wave intervals are being evaluated to identify periods of rapidrates and/or the presence of asystole. Upon detection of short R-waveintervals simultaneously in both ECG leads, indicative of an event that,if confirmed, may require the delivery of therapy, the device 14transitions from the not concerned state 302 to a concerned state 304,described in more detail herein below. In the concerned state 304 thedevice 14 evaluates a predetermined window of ECG signals to determinethe likelihood that the signal is corrupted with noise and todiscriminate rhythms requiring shock therapy from those that do notrequire shock therapy, using a combination of R-wave intervals and ECGsignal morphology information.

If a rhythm requiring shock therapy continues to be detected while inthe concerned state 304, the device 14 transitions from the concernedstate 304 to an armed state 306, described in more detail herein below.If a rhythm requiring shock therapy is no longer detected while thedevice is in the concerned state 304 and the R-wave intervals aredetermined to no longer be short, the device 14 returns to the notconcerned state 302. However, if a rhythm requiring shock therapy is nolonger detected while the device is in the concerned state 304, but theR-wave intervals continue to be detected as being short, processingcontinues in the concerned state 304.

In the armed state 306, the device 14 charges the high voltage shockingcapacitors and continues to monitor R-wave intervals and ECG signalmorphology for spontaneous termination. If spontaneous termination ofthe rhythm requiring shock therapy occurs, the device 14 returns to thenot concerned state 302. If the rhythm requiring shock therapy is stilldetermined to be occurring once the charging of the capacitors iscompleted, the device 14 transitions from the armed state 306 to a shockstate 308, described in more detail herein below.

In the shock state 308, the device 14 delivers a shock and returns tothe armed state 306 to evaluate the success of the therapy delivered.

FIGS. 7A-7I are flow charts of a method for detecting arrhythmias in asubcutaneous device according to an embodiment of the present invention.As illustrated in FIG. 7A, device 14 continuously evaluates the twochannels ECG1 and ECG2 associated with two predetermined electrodevectors to when sensed events occur. For example, the electrode vectorsfor the two channels ECG1 and ECG2 may include a horizontal vectorselected between two of the electrodes 28 (ECG2) located along thehousing of the device 14 as one electrode vector, while the otherelectrode vector is a front to back vector selected between the distalelectrode 26 (ECG1) and one of the subcutaneous electrodes 28, forexample. The input signal from each channel ECG1 and ECG2 ispre-processed and rectified, and an adaptive auto-adjusting threshold isthen applied. According to an embodiment of the present invention, asensed event is determined to have occurred, for example, whenever therising edge of the filtered ECG crosses the threshold.

FIG. 8 is a graphical representation of sensing of cardiac activityaccording to an embodiment of the present invention. In particular, thepresent invention utilizes an adaptive auto-adjusting threshold 401during the R-wave sensing of Block 322 that includes a first thresholdlevel 403, a second threshold level 405, a third threshold level 407 anda fourth threshold level 409. An example of an auto-adjusting thresholdis described, for example, in commonly assigned U.S. Patent ApplicationPublication No. 2004/0049120, to Cao et al., filed Sep. 11, 2002,incorporated herein by reference in its entirety. Once there is a sensedevent, which occurs whenever the rising edge of the rectified filteredECG 411 crosses the threshold level, in this case threshold 403,indicated by marker 410, the threshold 401 is adjusted to the secondthreshold level 405, which is a first predetermined percentage of a peakamplitude 412 of the rectified filtered ECG 411, such as 65 percent, forexample.

A blanking period 414 (nominally 150 ms, for example) prevents doublecounting of R-waves. During blanking period 414, the threshold 401continues to track the predetermined percentage of rectified filteredECG 411 until peak 412 is detected. Threshold 401 is held at the secondthreshold level 405 during a threshold hold time period 416 (nominally100 ms, for example) starting from the peak 412 location to preventT-wave oversensing by delaying the linear decay. Threshold 401 thendecays at a first predetermined rate, such as 35% of peak 412 persecond, for example, until threshold 401 reaches the third thresholdlevel 407, which is a second predetermined percentage of peak amplitude412 (nominally 30%, for example). Threshold 401 is held at the thirdthreshold level 407 until a step drop time 418 from the sensed event 410(1.5 sec, for example) has expired. Once the step drop time 418 hasexpired, the threshold 401 is instantaneously set at the fourththreshold level 409 and begins to decay at a second predetermined rate,such as 20% of peak 412 per second, for example. The threshold 401continues to decay linearly at the second predetermined rate until thethreshold 401 reaches the first threshold level 403. At no time can thethreshold 401 become less than the first threshold level 403.

The step drop time 418 allows abrupt adjustment of the threshold 401 inorder to accommodate sensing of sudden reductions in R-wave amplitudes.The second predetermined rate associated with the linear decay is set ata rate that prevents oversensing of P-waves while maintaining adequatedecay for sensing sudden drops in R-waves. If, at any time throughoutthis threshold adjustment process, a sensed event re-occurs outsideblanking period 414, then the threshold 401 is adjusted to the secondthreshold level 405, and the threshold adjustment process is repeated.

According to an embodiment of the present invention, the nominalsettings for the R-wave detector parameters may be set, for example,with the first threshold level being 25 microvolts, the second thresholdlevel, third threshold level and fourth threshold level being set as 65,30 and 20 percent of the peak amplitude 412, respectively, blankingperiod 414 being set as 150 milliseconds, threshold hold time 416 beingset as 100 milliseconds, and a maximum threshold level being 650microvolts. These nominal settings may differ between the anteriorhousing-based bipolar ECG and the front to back ECG in order to accountfor the expected difference in amplitude and noise characteristics forthose vectors.

The R-wave sensing described above is applied to each ECG channel ECG1and ECG2 independently. According to the present invention, sensing ofventricular events on either channel will trigger execution of statemachine in states 1 and 4. During states 2 and 3, R-wave sensingcontinues but state machine is triggered every predetermined number ofseconds, as described below.

Returning to FIG. 7A, a buffer of the most recent 12 R-R intervalsobtained during R-wave processing using the sensing scheme of FIG. 8,described above, for example, is independently maintained for each ofthe two sensing channels ECG1 and ECG2. When the next sensed R-wave isobtained, Block 322, which initially would be the 12^(th) R-waveinterval, a heart rate estimate is determined, Block 323, using a metricof heart rate, such as the mean, trimmed mean, or median of the RRintervals, for example. According to an embodiment of the presentinvention, the 9^(th) fastest beat of the 12 beats on a beat by beatbasis is utilized as the heart rate metric. Using the 9^(th) fastestbeat provides an estimate of heart rate that is less susceptible tooversensing while maintaining reasonable sensitivity to short R-Rintervals as in the case of VT/VF. If the buffer of 12 R-R intervalscontains any unknown R-R intervals (i.e., because the buffer is not yetfilled) the initial estimate of heart rate is unknown.

Once the heart rate estimate is obtained using the heart rate metric, adetermination is made as to whether asystole is detected for eitherchannel, ECG1 or ECG 2, Block 324. According to an embodiment of thepresent invention, asystole is detected for the channel, for example,either by determining whether one of the 12 R-R intervals is greaterthan a predetermined time period, such as three seconds, for example, orif the time since the most recently sensed R wave exceeds apredetermined time period, such as three seconds, for example. Thelatter can occur if an R-wave is sensed, for example, in one channelECG1, but the other channel ECG2 has not had an R-wave sense in three ormore seconds. If asystole is detected for either of the two channelsECG1 or ECG2, the current 12 R-R intervals for channels that aredetermined to be in asystole are cleared from the buffers, Block 325,and the process continues by determining whether the current heart rateestimate is reliable for both channels ECG1 and ECG2, Block 328,described below.

If asystole is not detected for either channel ECG1 and ECG2, NO inBlock 324, a determination is made independently for both channels ECG1and ECG2 as to whether the current heart rate estimate for both channelsECG1 and ECG2 is reliable, Block 328. According to an embodiment of thepresent invention, the current heart rate estimate for each of the twochannels ECG1 and ECG2 is determined not to be reliable, No in Block328, if either there are unknown or cleared entries in the buffer forthat channel, or if a predetermined number of the sensed R-wavesassociated with the current 12 R-R intervals for that channel was sensedat the minimum sensing threshold level, i.e., the first threshold level403 of FIG. 8, for example, and if the current heart rate estimate forthe channel is less than the programmed heart rate threshold. Accordingto one embodiment, the predetermined number of sensed R-waves that mustbe sensed at the minimum threshold is set at two, for example. Inaddition, the programmed heart rate threshold may be within a range of150 to 240 beats per minute, and is nominally set at 180 beats perminute, for example. It is understood that while the processing isdescribed using a buffer of 12 R-R intervals, any number of intervalsand predetermined number of sensed R-waves that must be sensed at theminimum threshold may be utilized.

If the above analysis does not determine that both of the channels arereliable, No in Block 328, a determination is made as to whether justone of the channels was unreliable or if both channels were unreliable,Block 330. If both channels are determined to be unreliable, the current12 R-R intervals for both channels ECG1 and ECG2 are cleared from thebuffers, Block 326, and the next R-sense is obtained for each channel,Block 322 using the sensing scheme of FIG. 8, described above, so that anew heart rate estimate is determined, Block 323, based on the new R-Rintervals.

If only one channel is determined to be unreliable, the value for theheart rate estimate for both channels is set to the current heart rateestimate for the channel determined to be reliable, Block 332. Onceeither both channels are determined to be reliable, YES in Block 328, oronly one of the two channels is determined to be unreliable andtherefore the heart rate estimate for both channels is set to thecurrent heart rate estimate for the channel determined to be reliable,Block 332, the final heart rate estimate is determined for each channelECG1 and ECG2 based on those results, Block 334, i.e., the heart rateestimate for each channel is set equal to their respective heart rateestimates determined in Block 323, or both are set equal to the heartrate estimate associated with the channel determined to be reliable,Block 332. A determination is then made as to whether the final heartrate estimates for both channels is greater than a predetermined VT/VFthreshold, Block 336. According to an embodiment of the presentinvention, the predetermined VT/VF threshold of Block 336 is set at 180bpm, for example, although any desired threshold could be utilized.

If the final heart rate estimates for one or both channels is notgreater than the predetermined VT/VF threshold, the buffer containingthe 12 R-R intervals for the channel not greater than the predeterminedVT/VF threshold is updated by removing the first R-sense, shifting theremaining eleven R-sense samples back so that the second R-sense becomesthe first R-sense, and so forth, and inserting the next detectedR-sense, Block 322, as the twelfth R-sense for each correspondingchannel ECG1 and ECG2. A new current heart rate estimate is thendetermined, Block 323. Once the final heart rate estimates for bothchannels is greater than the predetermined VT/VF threshold, Yes in Block336, the process transitions from the not concerned state 302 to theconcerned state 304.

According to the present invention, upon transition from the notconcerned state 302 to the concerned state 304, Block 305, a most recentwindow of ECG data from both channels ECG1 and ECG2 are utilized, suchas three seconds, for example, so that processing is triggered in theconcerned state 304 by a three-second timeout, rather than by thesensing of an R-wave, which is utilized when in the not concerned state302, described above. It is understood that while the processing isdescribed as being triggered over a three second period, other timesperiods for the processing time utilized when in the concerned state 304may be chosen, but should preferably be within a range of 0.5 to 10seconds. As a result, although sensing of individual R-waves continuesto occur in both channels ECG1 and ECG2 when in the concerned state 304,and the buffer of 12 R-R intervals continues to be updated, theopportunities for changing from the concerned state 304 to another stateand the estimates of heart rate only occur once the three-second timerexpires. Upon initial entry to the concerned state 304, it isadvantageous to process the most recent three-seconds of ECG data, i.e.,ECG data for the three seconds leading up to the transition to theconcerned state 304. This requires a continuous circular buffering ofthe most recent three seconds of ECG data even while in the notconcerned state 302.

As described in detail below, while in the concerned state 304, thepresent invention determines how sinusoidal and how noisy the signalsare in order to determine the likelihood that a ventricular fibrillation(VF) or fast ventricular tachycardia (VT) event is taking place, sincethe more sinusoidal and low noise the signal is, the more likely a VT/VFevent is taking place. As illustrated in FIG. 7B, once the devicetransitions from the not concerned state 302 to the concerned state 304,Block 305, a buffer for each of the two channels ECG 1 and ECG2 forstoring classifications of 3-second segments of data as “shockable” or“non-shockable” is cleared. Processing of signals of the two channelsECG1 and ECG2 while in the concerned state 304 is then triggered by thethree second time period, rather than by the sensing of an R-waveutilized during the not concerned state 302, described above.

Once the three second time interval has expired, YES in Block 341,morphology characteristics of the signal during the three second timeinterval for each channel are utilized to determine whether the signalsare likely corrupted by noise artifacts and to characterize themorphology of the signal as “shockable” or “not shockable”. For example,using the signals associated with the three second time interval, adetermination is made for each channel ECG1 and ECG 2 as to whether thechannel is likely corrupted by noise, Block 342, and a determination isthen made as to whether both channels ECG1 and ECG2 are corrupted bynoise, Block 344.

As illustrated in FIG. 7C, the determination as to whether the signalassociated with each of the channels ECG1 and ECG2 is likely corruptedby noise, Block 342 of FIG. 7B, includes multiple sequential noise teststhat are performed on each channel ECG and ECG2. During a first noisetest, for example, a determination is made as to whether a metric ofsignal energy content of the signal for the channel is withinpredetermined limits, Block 380. For example, the amplitude of eachsample associated with the three second window is determined, resultingin N sample amplitudes, from which a mean rectified amplitude iscalculated as the ratio of the sum of the rectified sample amplitudes tothe total number of sample amplitudes N for the segment. If the samplingrate is 256 samples per second, for example, the total number of sampleamplitudes N for the three-second segment would be N=768 samples.

Once the mean rectified amplitude is calculated, a determination is madeas to whether the mean rectified amplitude is between an upper averageamplitude limit and a lower average amplitude limit, the lower averageamplitude limit being associated with asystole episodes without artifactand the upper average amplitude limit being associated with a valuegreater than what would be associated with ventricular tachycardia andventricular fibrillation events. According to an embodiment of thepresent invention, the upper average amplitude limit is set as 1.5 mV,and the lower average amplitude limit is set as 0.013 mV. While themetric of signal energy content is described above as the mean rectifiedamplitude, it is understood that other signal of energy contents couldbe utilized.

If the determined mean rectified amplitude is not between the upperaverage amplitude limit and the lower average amplitude limit, the threesecond segment for that channel is identified as being likely corruptedwith noise, Block 386, and no further noise tests are initiated for thatchannel's segment.

If the determined mean rectified amplitude is located between the upperaverage amplitude limit and the lower average amplitude limit, a noiseto signal ratio is calculated and a determination is made as to whetherthe noise to signal ratio is less than a predetermined noise to signalthreshold, Block 382. For example, the amplitude of each sampleassociated with the three second window is determined, resulting in Nraw sample amplitudes. The raw signal is lowpass filtered, resulting inL lowpass sample amplitudes. The raw mean rectified amplitude isdetermined as the average of the absolute values of the raw sampleamplitudes. The lowpass mean rectified amplitude is determined as theaverage of the absolute values of the lowpass sample amplitudes. Next, ahighpass mean rectified amplitude is then calculated as the differencebetween the raw mean rectified amplitude and the lowpass mean rectifiedamplitude. The noise to signal ratio is then determined as the ratio ofthe highpass mean rectified amplitude to the lowpass mean rectifiedamplitude. If the noise to signal ratio is greater than a predeterminedthreshold, such as 0.0703, for example, the three second segment forthat channel is identified as being likely corrupted with noise, Block386, and no further noise tests are initiated for the segment.

If the noise to signal ratio is less than or equal to the predeterminedthreshold, a determination is made as to whether the signal is corruptedby muscle noise, Block 384. According to an embodiment of the presentinvention, the determination as to whether the signal is corrupted bymuscle noise is made by determining whether the signal includes apredetermined number of signal inflections indicative of the likelihoodof the signal being corrupted by muscle noise, using a muscle noisepulse count that is calculated to quantify the number of signalinflections in the three second interval for each channel ECG1 and ECG2.The presence of a significant number of inflections is likely indicativeof muscle noise.

FIG. 9A is a graphical representation of a determination of whether asignal is corrupted by muscle noise according to an embodiment of thepresent invention. FIG. 9B is a flowchart of a method of determiningwhether a signal is corrupted by muscle noise according to an embodimentof the present invention. For example, as illustrated in FIGS. 9A and9B, in order to determine a muscle noise count for the three secondinterval, the raw signal 420 is applied to a first order derivativefilter to obtain a derivative signal 422, and all of the zero-crossings424 in the derivative signal 422 are located, Block 460. A data paircorresponding to the data points immediately prior to and subsequent tothe zero crossings 424, points 426 and 428 respectively, for eachcrossing is obtained. The value of the data point in each data pair withsmaller absolute value is zeroed in order to allow a clear demarcationof each pulse when a rectified signal 430 is derived from the derivativesignal 422 with zeroed zero-crossing points 432.

A pulse amplitude threshold Td, for determining whether the identifiedinflection is of a significant amplitude to be identified as beingassociated with muscle noise, is determined, Block 462, by dividing therectified signal from the three second segment into equal sub-segments434, estimating a local maximum amplitude 436-442 for each of thesub-segments 434, and determining whether the local amplitudes 436-442are less than a portion of the maximum amplitude, which is maximumamplitude 440 in the example of FIG. 9, for the whole three secondsegment. If the local maximum amplitude is less than the portion of themaximum amplitude for the whole three second segment, the local maximumamplitude is replaced by the maximum for the whole three second segmentfor the sub-segment corresponding to that local maximum amplitude.

It is understood that while only two or less zero-crossing points areshown as being located within the sub-segments in the illustration ofFIG. 9 for the sake of simplicity, in fact each of the sub-segments 434,which have a length of approximately 750 milliseconds, will contain manyinflections, such as every 25 milliseconds, for example.

According to an embodiment of the present invention, the three secondsegment is divided into four sub-segments and the local maximumamplitudes are replaced by the maximum amplitude for the whole segmentif the local maximum amplitude is less than one fifth of the maximumamplitude for the whole segment. Once the determination of whether toreplace the local maximum amplitudes for each of the sub-segments withthe maximum amplitude for the whole segment is completed, the pulseamplitude threshold Td for the segment is set equal to a predeterminedfraction of the mean of the local maximum amplitudes for each of thesub-segments. According to an embodiment of the present invention, thepulse amplitude threshold Td for the three second segment is set equalto one sixth of the mean of the local maximum amplitudes 436-440.

Once the pulse amplitude threshold Td has been determined, theinflections associated with the signal for the three second segment isclassified as being of significant level to be likely indicative ofnoise by determining whether the pulse amplitude threshold Td is lessthan a pulse threshold, Block 464. According to an embodiment of thepresent invention, the pulse threshold is set as 1 microvolt. If thepulse amplitude threshold Td is less than the pulse threshold, thesignal strength is too small for a determination of muscle noise, andtherefore the signal is determined to be not likely corrupted by noiseand therefore the channel is determined to be not noise corrupted, Block466.

If the pulse amplitude threshold Td is greater than or equal to thepulse threshold, the three second segment is divided into twelvesub-segments of 250 ms window length, the number of muscle noise pulsesin each sub-segment is counted, and both the sub-segment having themaximum number of muscle noise pulses and the number of sub-segmentshaving 6 or more muscle noise pulses that are greater than apredetermined minimum threshold is determined. Muscle noise isdetermined to be present in the signal if either the maximum number ofmuscle noise pulses in a single sub-segment is greater than a noisepulse number threshold or the number of sub-segments of the twelvesub-segments having 6 or more muscle noise pulses greater than theminimum threshold is greater than or equal to a sub-segment pulse countthreshold. According to an embodiment of the present invention, thenoise pulse number threshold is set equal to eight and the sub-segmentpulse count threshold is set equal to three.

For example, if the pulse amplitude threshold Td is greater than orequal to the pulse threshold, No in Block 464, the maximum number ofmuscle noise counts in a single sub-segment is determined, Block 468. Ifthe maximum number of muscle noise counts is greater than the noisepulse number threshold, Yes in Block 470, the channel is determined tobe noise corrupted, Block 472. If the maximum number of muscle noisecounts for the channel is less than or equal to the noise pulse numberthreshold, No in Block 470, the number of sub-segments of the twelvesub-segments having 6 or more muscle noise pulses greater than theminimum threshold is determined, Block 474, and if the number is greaterthan or equal to a sub-segment pulse count threshold, Yes in Block 476,the channel is determined to be noise corrupted, Block 472. If thenumber is less than the sub-segment pulse count threshold, No in Block476, the channel is determined not to be noise corrupted, Block 466.

FIG. 9C is a flowchart of a method of determining whether a signal iscorrupted by muscle noise according to an embodiment of the presentinvention. Since muscle noise can be present during an episode ofventricular tachycardia, the width of the overall signal pulse waveformis determined in order to distinguish between signals that aredetermined likely to be purely noise related and signals that are bothshockable events and determined to include noise. Therefore, asillustrated in FIG. 9C, according to an embodiment of the presentinvention, once muscle noise is determined to be present as a result ofthe muscle noise pulse count being satisfied, No in Block 470 and Yes inBlock 476, a determination is made as to whether the signal is bothnoise corrupted and shockable, Block 480.

According to an embodiment of the present invention, the determinationin Block 480 as to whether the signal is both noisy and shockable ismade, for example, by dividing the rectified signal, having 768 datapoints, into four sub-segments and determining a maximum amplitude foreach of the four sub-segments by determining whether a maximum amplitudefor the sub-segment is less than a portion of the maximum amplitude forthe entire rectified signal in the three second segment. For example, adetermination is made for each sub-segment as to whether the maximumamplitude for the sub-segment is less than one fourth of the maximumamplitude for the entire rectified signal. If less than a portion of themaximum amplitude for the entire rectified signal in the three secondsegment, the maximum amplitude for the sub-segment is set equal to themaximum amplitude for the entire rectified signal.

A mean rectified amplitude for each of the sub-segments is determined bydividing the sum of the rectified amplitudes for the sub-segment by thenumber of samples in the sub-segment, i.e., 768÷4. Then the normalizedmean rectified amplitude for each sub-segment is determined by dividingthe mean rectified amplitude for each of the sub-segments by the peakamplitude for the sub-segment. The normalized mean rectified amplitudefor the three second segment is then determined as the sum of thenormalized mean rectified amplitudes for each sub-segment divided by thenumber of sub-segments, i.e., four.

Therefore, once muscle noise is suspected as a result of thedetermination of the muscle noise pulse count, the determination ofBlock 480 based on whether the normalized mean rectified amplitude forthe three second segment is greater than a predetermined threshold foridentifying signals that, despite being indicative of a likelihood ofbeing associated with noise, nevertheless are associated with ashockable event. For example, according to an embodiment of the presentinvention, a determination is made as to whether the normalized meanrectified amplitude for the three second segment is greater than 18microvolts. If the normalized mean rectified amplitude for the threesecond segment is less than or equal to the predetermined threshold, thechannel is likely corrupted by muscle noise and not shockable, No inBlock 480, and is therefore identified as being corrupted by noise,Block 472. If the normalized mean rectified amplitude for the threesecond segment is greater than the predetermined threshold, the channelis determined to be likely corrupted by muscle noise and shockable, Yesin Block 480, and is therefore identified as not to be likely corruptedby muscle noise, Block 478.

Returning to FIG. 7C, when the signal is determined to be not likelycorrupted by muscle noise, a determination is made as to whether themean frequency of the signal associated with the channel is less than apredetermined mean frequency threshold, Block 388, such as 11 Hz forexample. The mean frequency of the signal during the 3 second segmentfor each channel ECG 1 and ECG2 is generated, for example, bycalculating the ratio of the mean absolute amplitude of the firstderivative of the 3 second segment to the mean absolute amplitude of the3 second segment, multiplied by a constant scaling factor. If the meanfrequency is determined to be greater than or equal to the predeterminedmean frequency threshold, No in Block 388, the three second segment forthat channel is identified as being likely corrupted with noise, Block386. If the mean frequency is determined to be less than thepredetermined mean frequency threshold, Yes in Block 388, the threesecond segment for that channel is identified as being not noisecorrupted, Block 390.

According to an embodiment of the present invention, since the meanspectral frequency tends to be low for true ventricular fibrillationevents, moderate for organized rhythms such as sinus rhythm andsupraventricular tachycardia, for example, and high during asystole andnoise, the determination in Block 388 includes determining whether themean frequency is less than a predetermined upper mean frequencythreshold, such as 11 Hz (i.e., mean period T of approximately 91milliseconds) for example, and whether the mean frequency is less than apredetermined lower mean frequency, such as 3 Hz for example. If themean frequency is below a second, lower threshold, such as 3 Hz, forexample, the signal is also rejected as noise and no further noise testsare initiated. This comparison of the mean frequency to a second lowerthreshold is intended to identify instances of oversensing, resulting inappropriate transition to the concerned state. If the mean frequency ofthe signal is less than 3 Hz, it is generally not possible for the heartrate to be greater than 180 beats per minute. In practice, it may beadvantageous to set the lower frequency threshold equal to theprogrammed VT/VF detection rate, which is typically approximately 3 Hz.

Therefore, in the determination of Block 388, if the mean frequency isdetermined to be either greater than or equal to the predetermined uppermean frequency threshold or less than the lower threshold, the threesecond segment for that channel is identified as being likely corruptedwith noise, Block 386. If the mean frequency is determined to be bothless than the predetermined upper mean frequency threshold and greaterthan the lower threshold, the three second segment for that channel isidentified as not being noise corrupted, Block 390.

Returning to FIG. 7B, once the determination as to whether the channelsECG1 and ECG2 are corrupted by noise is made, Block 342, a determinationis made as to whether both channels are determined to be noisecorrupted, Block 344. If the signal associated with both channels ECG1and ECG2 is determined to likely be corrupted by noise, both channelsare classified as being not shockable, Block 347, and therefore a bufferfor each channel ECG1 and ECG 2 containing the last threeclassifications of the channel is updated accordingly. If both channelsECG1 and ECG2 are not determined to be likely corrupted by noise, No inBlock 344, the device distinguishes between either one of the channelsbeing not corrupted by noise or both channels being not corrupted bynoise by determining whether noise was determined to be likely in onlyone of the two channels ECG1 and ECG2, Block 346.

If noise was likely in only one of the two channels, a determination ismade whether the signal for the channel not corrupted by noise, i.e.,the clean channel, is more likely associated with a VT event or with aVF event by determining, for example, whether the signal for thatchannel includes R-R intervals that are regular and the channel can betherefore classified as being relatively stable, Block 348. If the R-Rintervals are determined not to be relatively stable, NO in Block 348,the signal for that channel is identified as likely being associatedwith VF, which is then verified by determining whether the signal is ina VF shock zone, Block 350, described below. If R-R intervals for thatchannel are determined to be stable, YES in Block 348, the signal isidentified as likely being associated with VT, which is then verified bydetermining whether the signal is in a VT shock zone, Block 352,described below.

If noise was not likely for both of the channels, No in Block 346, i.e.,both channels are determined to be clean channels, a determination ismade whether the signal for both channels is more likely associated witha VT event or with a VF event by determining whether the signal for bothchannels includes R-R intervals that are regular and can be thereforeclassified as being relatively stable, Block 356. If the R-R intervalsare determined not to be relatively stable, NO in Block 356, the signalfor both channels is identified as likely being associated with VF,which is then verified by determining whether the signal for eachchannel is in a VF shock zone, Block 360, described below. If R-Rintervals for both channels are determined to be stable, YES in Block356, the signal is identified as likely being associated with VT, whichis then verified by determining, based on both channels, whether thesignal is in a VT shock zone, Block 352.

As illustrated in FIG. 7D, according to an embodiment of the presentinvention, in order to determine whether the signal for both channelsincludes R-R intervals that are regular and the channels can betherefore classified as being relatively stable, Block 356,predetermined maximum and minimum intervals for each channel ECG1 andECG2 are identified, Block 500, from the updated buffer of 12RR-intervals, Block 342. According to one embodiment of the presentinvention, the largest RR-interval and the sixth largest RR-interval ofthe twelve RR-intervals are utilized as the maximum interval and theminimum interval, respectively.

The difference between the maximum RR-interval and the minimumRR-interval of the 12 RR-intervals is calculated for each channel ECG1and ECG2, Block 502, to generate a first interval difference associatedwith the first channel ECG1 and a second interval difference associatedwith the second channel ECG2. The smallest of the first intervaldifference and the second interval difference is then identified, Block504, and a determination is made as to whether the minimum of the firstinterval difference and the second interval difference is greater than apredetermined stability threshold, Block 506, such as 110 milliseconds,for example.

If the minimum of the first interval difference and the second intervaldifference is greater than the stability threshold, the event isclassified as an unstable event, Block 508, and a determination is madefor each channel as to whether the signal associated with the channel iswithin a predetermined VF shock zone, Blocks 360 and 362 of FIG. 7B,described below. If the minimum of the first interval difference and thesecond interval difference is less than or equal to the stabilitythreshold, No in Block 506, the device determines which one of theminimum RR-interval associated with the first channel ECG1 and theminimum RR-interval associated with the second channel ECG2 is shortest,Block 510, and determines whether the shortest minimum interval isgreater than a minimum interval threshold, Block 512, such as 200milliseconds, for example.

If the shortest of the two minimum intervals is less than or equal tothe minimum interval threshold, the event is classified as an unstableevent, Block 508, and a determination is made for each channel as towhether the signal associated with the channel is within a predeterminedVF shock zone, Blocks 360 and 362 of FIG. 7B, described below. If theshortest of the two minimum intervals is greater than the minimuminterval threshold, the device determines which one of the minimumRR-interval associated with the first channel ECG1 and the minimumRR-interval associated with the second channel ECG2 is the greatest,Block 514, and determines whether the maximum of the two minimumintervals is less than or equal to a maximum interval threshold, Block516, such as 333 milliseconds for example. If the maximum of the twominimum intervals is greater than the maximum interval threshold, theevent is classified as an unstable event, Block 508, and a determinationis made for each channel as to whether the signal associated with thechannel is within a predetermined VF shock zone, Blocks 360 and 362 ofFIG. 7B, described below. If the maximum of the two minimum intervals isless than or equal to the maximum interval threshold, the event isclassified as a stable event, Block 518, and a determination is made,based on both channels ECG1 and ECG2, as to whether the signal is withina predetermined VT shock zone, Block 358, described below.

Returning to FIG. 7B, according to an embodiment of the presentinvention, during the determination of whether the signal associatedwith each of the channels ECG1 and ECG2 is within the VF shock zone,Blocks 360 and 362, the VF shock zone is defined based upon a low slopecontent metric and a spectral width metric for each of the two channelsECG1 and ECG2. The low slope content metric is calculated as the ratioof the number of data points with low slope to the total number ofsamples in the 3-second segment. For example, according to an embodimentof the present invention, the difference between successive ECG samplesis determined as an approximation of the first derivative (i.e, theslope) of the ECG signal. In particular, as illustrated in FIG. 7E, theraw signal for each channel is applied to a first order derivativefilter to obtain a derivative signal for the three-second segment, Block530. The derivative signal is then rectified, divided into four equalsub-segments, and the largest absolute slope is estimated for each ofthe four sub-segments, Block 532.

A determination is made as to whether the largest absolute slopes areless than a portion of the overall largest absolute slope for the wholethree-second segment, Block 534, such as one-fifth of the overallabsolute slope, for example. If the largest absolute slope is less thanthe portion of the overall slope, then the slope value for thatsub-segment is set equal to the overall largest absolute slope, Block536. If the largest absolute slope is not less than the portion of theoverall slope, then the slope value for that sub-segment is set equal tothe determined largest absolute slope for the sub-segment, Block 538.

Once the slope value for each of the sub-segments has been determinedand updated by being set equal to the largest slope for the three secondsegment, if necessary, the average of the four slopes is calculated anddivided by a predetermined factor, such as 16 for example, to obtain alow slope threshold, Block 540. The low slope content is then obtainedby determining the number of sample points in the three-second segmenthaving an absolute slope less than or equal to the low slope threshold,Block 542.

According to an embodiment of the present invention, if, during thedetermination of the low slope threshold in Block 540, the low slopethreshold is a fraction, rather than a whole number, a correction ismade to the low slope content to add a corresponding fraction of thesamples. For example, if the threshold is determined to be 4.5, then thelow slope content is the number of sample points having an absoluteslope less than or equal to 4 plus one half of the number of samplepoints with slope equal to 5.

The spectral width metric, which corresponds to an estimate of thespectral width of the signal for the three-second segment associatedwith each channel ECG1 and ECG2, is defined, for example, as thedifference between the mean frequency and the fundamental frequency ofthe signal. According to an embodiment of the present invention, thespectral width metric is calculated by determining the differencebetween the most recent estimate of the RR-cycle length and the meanspectral period of the signal for that channel. As is known in the art,the mean spectral period is the inverse of the mean spectral frequency.

It is understood that R-R cycle length utilized in the concerned stateand armed state can be different than that used in the not concernedstate. For example, according to an embodiment of the present invention,the 9^(th) longest R-R interval is utilized in the not concerned stateand the mean of the 7^(th) to the 10^(th) R-R interval is utilized inthe concerned state and the armed state.

FIG. 10 is a graphical representation of a VF shock zone according to anembodiment of the present invention. As illustrated in FIG. 10, a VFshock zone 500 is defined for each channel ECG1 and ECG2 based on therelationship between the calculated low slope content and the spectralwidth associated with the channel. For example, the shock zone isdefined by a first boundary 502 associated with the low slope contentset for by the equation:Low slope content=−0.0013×spectral width+0.415  Equation 1and a second boundary 504 associated with the spectral width set forthby the equation:spectral width=200  Equation 2

As can be seen in FIG. 10, since noise 506 tends to have a relativelyhigher spectral width, and normal sinus rhythm 508 tends to have arelatively higher low slope content relative to VF, both noise 506 andnormal sinus rhythm 508 would be located outside the VF shock zone 500.

A determination is made for each channel ECG1 and ECG2 as to whether thelow slope content for that channel is less than both the first boundary502 and the spectral width is less than the second boundary 504, i.e.,the low slope content is less than −0.0013×spectral width+0.415, and thespectral width is less than 200. For example, once the event isdetermined to be associated with VF, i.e., the intervals for bothchannels are determined to be irregular, No in Block 356, adetermination is made that channel ECG1 is in the VF shock zone, Yes inBlock 360, if, for channel ECG1, both the low slope content is less thanthe first boundary 502 and the spectral width is less than the secondboundary 504. The three second segment for that channel ECG1 is thendetermined to be shockable, Block 363 and the associated buffer for thatchannel is updated accordingly. If either the low slope content for thechannel is not less than the first boundary 502 or the spectral width isnot less than the second boundary, the channel ECG1 is determined not tobe in the VF shock zone, No in Block 360, the three second segment forthat channel ECG1 is then determined to be not shockable, Block 365, andthe associated buffer is updated accordingly.

Similarly, a determination is made that channel ECG2 is in the VF shockzone, Yes in Block 362, if, for channel ECG2, both the low slope contentis less than the first boundary 502 and the spectral width is less thanthe second boundary 504. The three second segment for that channel ECG2is then determined to be shockable, Block 369 and the associated bufferfor that channel is updated accordingly. If either the low slope contentfor the channel is not less than the first boundary 502 or the spectralwidth is not less than the second boundary, the channel ECG2 isdetermined not to be in the VF shock zone, No in Block 362, the threesecond segment for that channel ECG2 is then determined to be notshockable, Block 367, and the associated buffer is updated accordingly.

FIGS. 11A and 11B are graphical representations of the determination ofwhether an event is within a shock zone according to an embodiment ofthe present invention. During the determination of whether the event iswithin the VT shock zone, Block 358 of FIG. 7B, the low slope contentand the spectral width is determined for each channel ECG1 and ECG2, asdescribed above in reference to determining the VF shock zone. Adetermination is made as to which channel of the two signal channelsECG1 and ECG2 contains the minimum low slope content and which channelof the two signal channels ECG 1 and ECG2 contains the minimum spectralwidth. A first VT shock zone 520 is defined based on the relationshipbetween the low slope content associated with the channel determined tohave the minimum low slope content and the spectral width associatedwith the channel determined to have the minimum spectral width. Forexample, according to an embodiment of the present invention, the firstVT shock zone 520 is defined by a boundary 522 associated with theminimum low slope content and the minimum spectral width set forth bythe equation:LSC=−0.004×SW+0.93  Equation 3A second VT shock zone 524 is defined based on the relationship betweenthe low slope content associated with the channel determined to have theminimum low slope content and the normalized mean rectified amplitudeassociated with the channel determined to have the maximum normalizedmean rectified amplitude. The normalized mean rectified amplitudes forthe two channels ECG1 and ECG2 utilized during the VT shock zone test isthe same as described above in reference to the noise determination ofBlock 343. For example, according to an embodiment of the presentinvention, the second VT shock zone 524 is defined by a second boundary526 associated with the relationship between the minimum low slope countand the maximum normalized mean rectified amplitude set forth by theequation:NMRA=68×LSC+8.16  Equation 4

If both the minimum low slope count is less than the first boundary 522,i.e., −0.004× minimum spectral width+0.93, and the maximum normalizedmean rectified amplitude is greater than the second boundary 526, i.e.,68× minimum low slope count+8.16, the event is determined to be in theVT shock zone, YES in Block 358, and both channels ECG1 and ECG2 aredetermined to be shockable, Block 357, and the associated buffers areupdated accordingly. If either the minimum low slope count is not lessthan the first boundary 522 or the maximum normalized mean rectifiedamplitude is not greater than the second boundary 526, the event isdetermined to be outside the VT shock zone, NO in Block 358, and bothchannels ECG1 and ECG2 are determined to be not shockable, Block 359.

As described, during both the VF shock zone test, Blocks 360 and 362,and the VT shock zone test, Block 358, the test results for each channelECG1 and ECG2 as being classified as shockable or not shockable arestored in a rolling buffer containing the most recent eight suchdesignations, for example, for each of the two channels ECG1 and ECG2that is utilized in the determination of Block 356, as described below.

If only one of the two channels ECG1 and ECG2 is determined to becorrupted by noise, Yes in Block 346, a determination is made whetherthe signal for the channel not corrupted by noise, i.e., the “cleanchannel”, is more likely associated with a VT event or with a VF eventby determining whether the signal for the clean channel includes R-Rintervals that are regular and can be therefore classified as beingrelatively stable, Block 348. If the R-R intervals are determined not tobe relatively stable, NO in Block 348, the signal for the clean channelis identified as likely being associated with VF, which is then verifiedby determining whether the signal for the clean channel is in a VF shockzone, Block 350, described below. If R-R intervals for the clean channelare determined to be stable, YES in Block 348, the signal is identifiedas likely being associated with VT, which is then verified bydetermining whether the signal for the clean channel is in a VT shockzone, Block 352.

According to an embodiment of the present invention, in order todetermine whether the signal for the clean channel includes R-Rintervals that are regular and the clean channel can be thereforeclassified as being either relatively stable, Yes in Block 348, orrelatively unstable, No in Block 348, the device discriminates VT eventsfrom VF events in Block 348 by determining whether the relative level ofvariation in the RR-intervals associated with the clean channel isregular. For example, as illustrated in FIG. 7H, predetermined maximumand minimum intervals for the clean channel are identified, Block 700,from the updated buffer of 12 RR-intervals, Block 342 of FIG. 7B.According to an embodiment of the present invention, the largestRR-interval and the sixth largest RR-interval of the twelve RR-intervalsare utilized as the maximum interval and the minimum interval,respectively.

The difference between the maximum RR-interval and the minimumRR-interval of the 12 RR-intervals is calculated to generate an intervaldifference associated with the clean channel, 702. A determination isthen made as to whether the interval difference is greater than apredetermined stability threshold, Block 704, such as 110 milliseconds,for example.

If the interval difference is greater than the stability threshold, theevent is classified as an unstable event, Block 706, and therefore theclean channel is determined not to include regular intervals, No inBlock 348, and a determination is made as to whether the signalassociated with the clean channel is within a predetermined VF shockzone, Block 350 of FIG. 7B, described below. If the interval differenceis less than or equal to the stability threshold, No in Block 704, thedevice determines whether the minimum RR interval is greater than aminimum interval threshold, Block 710, such as 200 milliseconds, forexample.

If the minimum interval is less than or equal to the minimum intervalthreshold, No in Block 710, the event is classified as an unstableevent, Block 706, and therefore the clean channel is determined not toinclude regular intervals, No in Block 348, and a determination is madeas to whether the signal associated with the clean channel is within apredetermined VF shock zone, Block 350 of FIG. 7B, described below. Ifthe minimum interval is greater than the minimum interval threshold, Yesin Block 710, the device determines whether the maximum interval is lessthan or equal to a maximum interval threshold, Block 712, such as 333milliseconds for example. If the maximum interval is greater than themaximum interval threshold, the event is classified as an unstableevent, Block 706, and therefore the clean channel is determined not toinclude regular intervals, No in Block 348, and a determination is madeas to whether the signal associated with the clean channel is within apredetermined VF shock zone, Block 350 of FIG. 7B, described below. Ifthe maximum interval is less than or equal to the maximum intervalthreshold, the event is classified as a stable event, Block 714, andtherefore the clean channel is determined to include regular intervals,Yes in Block 348, and a determination is made as to whether the signalassociated with the clean channel is within a predetermined VT shockzone, Block 352 of FIG. 7B, described below.

Returning to FIG. 7B, the determination of whether the clean channel iswithin the VF shock zone, Block 350, is made based upon a low slopecontent metric and a spectral width metric, similar to the VF shock zonedetermination described above in reference to Blocks 360 and 362, bothof which are determined for the clean channel using the method describedabove. Once the low slope content metric and a spectral width metric aredetermined for the clean channel, the determination of whether the cleanchannel is in the VF shock zone is made using Equations 1 and 2, so thatif either the low slope content for the clean channel is not less thanthe first boundary 502 or the spectral width is not less than the secondboundary 504, the clean channel is determined not to be in the VF zone,No in Block 350 and both channels are classified as not shockable, Block351, and the associated buffers are updated accordingly.

If the low slope content for the clean channel is less than the firstboundary 502 and the spectral width is less than the second boundary504, the clean channel is determined to be in the VF zone, Yes in Block350. A determination is then made as to whether the channel determinedto be corrupted by noise, i.e., the “noisy channel”, is within the VFshock zone, Block 354. If either the low slope content for the noisychannel is not less than the first boundary 502 or the spectral width isnot less than the second boundary 504, the noisy channel is determinednot to be in the VF zone, No in Block 354, the clean channel isclassified as shockable and the noisy channel is classified as notshockable, Block 355, and the associated buffers are updatedaccordingly.

If the low slope content for the noisy channel is less than the firstboundary 502 and the spectral width is less than the second boundary504, the noisy channel is determined to be in the VF zone, Yes in Block354, both the clean channel and the noisy channel are classified asbeing shockable, Block 353, and the associated buffers are updatedaccordingly.

Similar to the VT shock zone determination described above in referenceto Block 358, during the determination as to whether the clean channelis within the VT shock zone in Block 352, the low slope content and thespectral width is determined for the clean channel as described above inreference to determining the VF shock zone. The first VT shock zone 520is defined based on the relationship between the low slope content andthe spectral width associated with the clean channel according toEquation 3, for example, and the second VT shock zone 524 is definedbased on the relationship between the low slope count and the normalizedmean rectified amplitude associated with the clean channel. Thenormalized mean rectified amplitudes for the clean channel is the sameas described above in reference to the noise detection tests of Block344. For example, according to an embodiment of the present invention,the second VT shock zone 524 is defined by a second boundary 526associated with the relationship between the low slope count and thenormalized mean rectified amplitude of the clean channel using Equation4.

If both the low slope count is less than the first boundary 522, i.e.,−0.004×spectral width of clean channel+0.93, and the normalized meanrectified amplitude is greater than the second boundary 526, i.e., 68×low slope count of clean channel+8.16, the clean channel is determinedto be in the VT shock zone, Yes in Block 352, both channels areclassified as being shockable, Block 353, and the associated buffers areupdated accordingly.

If either the low slope count is not less than the first boundary 522 orthe maximum normalized mean rectified amplitude is not greater than thesecond boundary 526, the clean channel is determined to be outside theVT shock zone, No in Block 352, both channels are classified as beingnot shockable, Block 351, and the associated buffers are updatedaccordingly.

Once the classification of both of the channels ECG1 and ECG2 is madesubsequent to the determination of whether the clean channel or channelsis in the VT shock zone, Block 352 and 358, or the VF shock zone, Blocks350 and Blocks 360 and 362 in combination, a determination is made as towhether the device should transition from the concerned state 304 to thearmed state 306, Block 370. For example, according to an embodiment ofthe present invention, the transition from the concerned state 304 tothe armed state 306 is confirmed if a predetermined number, such as twoout of three for example, of three-second segments for both channelsECG1 and ECG2 have been classified as being shockable. If thepredetermined number of three-second segments in both channels ECG1 andECG2 have been classified as shockable, the device transitions from theconcerned state 304 to the armed state 306, Yes in Block 370. If thepredetermined number of three-second segments in both channels ECG1 andECG2 have not been classified as shockable, the device does nottransition from the concerned state 304 to the armed state 306, no inBlock 370, and a determination as to whether to transition back to thenot concerned state 302 is made, Block 372. The determination as towhether to transition from the concerned state 304 back to the notconcerned state 302 is made, for example, by determining whether theheart rate estimate is less than a heart rate threshold level in both ofthe two channels ECG1 and ECG2. If it is determined that the deviceshould not transition to the not concerned state 302, i.e., either ofthe two heart rate estimates are greater than the heart rate threshold,the process is repeated using the signal generated during a nextthree-second window, Block 341.

According to an embodiment of the present invention, the heart ratethreshold level is set as 180 bpm, for example, and a single estimate ofheart rate (that occurs every three seconds) that is greater than theheart rate threshold level in both channels will suffice to cause thedevice to transition from the concerned state 304 to the not concernedstate 302, Yes in Block 372.

When the device transitions from the concerned state 304 to the armedstate 306, Yes in Block 370, processing continues to be triggered by athree-second time out as is utilized during the concerned state 304,described above. As illustrated in FIG. 7F, once the device transitionsfrom the concerned state 302 to the armed state 306, charging of thecapacitors is initiated, Block 600. During the charging of thecapacitors, the classification of segments for each channel ECG1 andECG2 as being either shockable or not shockable generated during theshock zone tests described above continues and once the next threeseconds of data has been acquired, Block 601, a determination is made aswhether the event continues to be a shockable event by determiningwhether a predetermined number of segments, such as the most recent twosegments for example, have been classified in both of the two channelsECG1 and ECG2 as not shockable, Block 602. If the predetermined numberof three second segments have been classified as not shockable,indicating that the event may possibly no longer be a shockable event,Yes in Block 602, the charging of the capacitors is stopped, Block 604,and a determination is made as to whether to transition to the notconcerned state 302, Block 606.

According to an embodiment of the present invention, the device willtransition from the armed state 306 to the not concerned state 302, Yesin Block 606, if certain termination requirements are met. For example,return to the not concerned state 302 occurs if, for both channels ECG1and ECG2 simultaneously, less than two out of the last threethree-second segments are classified as shockable, less than three outof the last eight three-second segments are classified as shockable, andthe most recent three second segment is classified as not shockable.Another possible criteria for returning to the not concerned state 302is the observation of 4 consecutive not shockable classifications inboth channel ECG1 and ECG2 simultaneously.

In addition to the two criteria described above, at least one of thecurrent heart rate estimates must be slower than the programmed ratethreshold 403, and capacitor charging must not be in progress. If eachof these requirements are satisfied, Yes in Block 606, the devicetransitions from the armed state 306 to the not concerned state 302.

If one or more of these requirements are determined not to be satisfied,return to the not concerned state is not indicated, No in Block 606, anda determination is then made as whether the shockable rhythm isredetected, Block 608, by determining whether predetermined redetectionrequirements have been satisfied. For example, a determination is madeas to whether a predetermined number of three-second segments in both ofthe two channels ECG1 and ECG2, such as two out of the most recent threefor example, have been classified as being shockable. If thepredetermined redetection requirements are not satisfied, No in Block608, the determination of whether to terminate delivery of the therapy,Block 606, is repeated so that the processing switches between thedetermination of whether to terminate delivery of therapy, Block 606 andthe determination as to whether the shockable event is redetected, Block608, until either the event has terminated and the device transitionsfrom the armed state 306 to the not concerned state 302 or the event isredetected. If the predetermined redetection requirements are met, Yesin Block 608, charging is re-initiated, Block 600, and the process isrepeated.

If, during the charging of the capacitors, the predetermined number ofthree second segments have not been classified as not shockable, No inBlock 602, a determination is made as to whether the charging of thecapacitors is completed, Block 610. As long as the capacitor chargingcontinues to occur, No in Block 602, once the charging of the capacitorsis completed, Yes in Block 610, a determination is made as to whetherdelivery of the therapy is still appropriate, Block 612, by determiningwhether predetermined therapy delivery confirmation requirements havebeen satisfied. For example, according to an embodiment of the presentinvention, the predetermined therapy delivery confirmation requirementsinclude determining whether, for both channels ECG1 and ECG2, at leastfive out of the last eight three-second segments are classified as beingshockable, and at least two of the last three three-second segments areclassified as being shockable. In addition, a determination is made asto whether the most recent three-second segment has been classified asbeing shockable for at least one of the two channels ECG1 and ECG2.

If the predetermined therapy delivery requirements have not beensatisfied, and therefore the delivery of the therapy is not confirmed,No in Block 612, the determination of whether to transition from thearmed state 306 to the not concerned state 302, Block 606, is repeated.If the predetermined therapy delivery requirements are satisfied, andtherefore the delivery of the therapy is confirmed, Yes in Block 612,the device transitions from the armed state 306 to the shock state 308.

As illustrated in FIG. 7G, once the device transitions from the armedstate 306 to the shock state 308, the therapy is delivered uponobservation of the first sensed R-wave, Block 630, the episode data isstored, Block 632, and the buffers for storing the eight three secondsegments are cleared, Block 634. Once a post shock timer, such as threeseconds for example, has expired, Yes in Block 636, the devicetransitions from the shock state 308 to Block 606 of the armed state306. Since, as described above, classification of at least threesubsequent three-second segments is required before the terminationdecision can be made in Block 606 subsequent to the delivery of therapyin the shock state 308, a determination based on the terminationrequirements cannot be initiated until at least twelve seconds after theinitial shock therapy was delivered. The termination and redetectionrequirements are then reviewed until one of the two requirements aresatisfied, i.e., the event is determined to have terminated, Yes inBlock 606, or the event is redetected, Yes in Block 608. If theredetection requirements are satisfied, the charging of the capacitorsis again initiated, Block 600, and processing in the armed state 306continues as described above until all available therapies have beenexhausted.

It will be apparent from the foregoing that while particular embodimentsof the invention have been illustrated and described, variousmodifications can be made without departing from the spirit and scope ofthe invention. Accordingly, it is not intended that the invention belimited, except as by the appended claims. For example, as illustratedin FIG. 7I, during the noise determination of Block 744, thedetermination is made for each channel ECG1 and ECG2 as to whether thechannel is corrupted by noise as described above. However, according toan embodiment of the present invention, once noise is determined to bepresent in either channel, No in Blocks 380, 382 or 388, Yes in Block384 of FIG. 7C, both channels are classified as being not shockable,Block 748.

If noise is not present in either channel ECG1 and ECG2, No in Block744, a determination is made as for each channel ECG1 and ECG2 as towhether the channel is in a VF shock zone. For example, according to anembodiment of the present invention, a determination is made thatchannel ECG1 is in the VF shock zone, Yes in Block 748, if, for channelECG1, both the low slope content is less than the first boundary 502 andthe spectral width is less than the second boundary 504, as describedabove. The three second segment for that channel ECG1 is then determinedto be shockable, Block 750 and the associated buffer for that channel isupdated accordingly. If either the low slope content for the channel isnot less than the first boundary 502 or the spectral width is not lessthan the second boundary, the channel ECG1 is determined not to be inthe VF shock zone, No in Block 748, the three second segment for thatchannel ECG1 is then determined to be not shockable, Block 752, and theassociated buffer is updated accordingly.

Similarly, a determination is made that channel ECG2 is in the VF shockzone, Yes in Block 754, if, for channel ECG2, both the low slope contentis less than the first boundary 502 and the spectral width is less thanthe second boundary 504, as described above. The three second segmentfor that channel ECG2 is then determined to be shockable, Block 756 andthe associated buffer for that channel is updated accordingly. If eitherthe low slope content for the channel is not less than the firstboundary 502 or the spectral width is not less than the second boundary,the channel ECG2 is determined not to be in the VF shock zone, No inBlock 754, the three second segment for that channel ECG2 is thendetermined to be not shockable, Block 758, and the associated buffer isupdated accordingly.

Once the classification of both of the channels ECG1 and ECG2 as beingeither shockable, Block 752 and 758, or not shockable, Blocks 748, 754and 760, a determination is made as to whether the device shouldtransition from the concerned state 304 to the armed state 306, Block762. The determination of whether the device should transition from theconcerned state 304 to the armed state 306 in Block 762, in addition tothe subsequent determination of whether to transition from the concernedstate 304 to the not concerned state 302 in Block 764 are similar to thedetermination of whether the device should transition from the concernedstate 304 to the armed state 306 in Block 370, and to the determinationof whether to transition from the concerned state 304 to the notconcerned state 302 in Block 372 in FIG. 7B described above, andtherefore will not be repeated for the sake of brevity.

Some of the techniques described above may be embodied as acomputer-readable medium comprising instructions for a programmableprocessor such as microprocessor 142, pacer/device timing circuit 178 orcontrol circuit 144 shown in FIG. 3. The programmable processor mayinclude one or more individual processors, which may act independentlyor in concert. A “computer-readable medium” includes but is not limitedto any type of computer memory such as floppy disks, conventional harddisks, CD ROMS, Flash ROMS, nonvolatile ROMS, RAM and a magnetic oroptical storage medium. The medium may include instructions for causinga processor to perform any of the features described above forinitiating a session of the escape rate variation according to thepresent invention.

While a particular embodiment of the present invention has been shownand described, modifications may be made. It is therefore intended inthe appended claims to cover all such changes and modifications, whichfall within the true spirit and scope of the invention.

1. A method of treating a cardiac event with a medical device having therapeutic componentry and a plurality of electrodes, comprising: sensing cardiac signals from the plurality of electrodes, the plurality of electrodes forming a first sensing vector and a second sensing vector; determining a first characteristic of the cardiac signals sensed by the first sensing vector over a predetermined sensing window and a first characteristic of the cardiac signals sensed by the second sensing vector over the predetermined sensing window; determining a second characteristic of the cardiac signals sensed by the first sensing vector different from the first characteristic of the first sensing vector over the predetermined sensing window and a second characteristic of the cardiac signals sensed by the second sensing vector different from the first characteristic of the second sensing vector over the predetermined sensing window; determining a third characteristic of the cardiac signals sensed by the first sensing vector different from the first characteristic of the first sensing vector and the second characteristic of the first sensing vector over the predetermined sensing window and a third characteristic of the cardiac signals sensed by the second sensing vector different from the first characteristic of the second sensing vector and the second characteristic of the second sensing vector over the predetermined sensing window; delivering a therapy from the therapeutic componentry based at least in part, on: a relationship of one of the first characteristic sensed by the first sensing vector and the first characteristic sensed by the second sensing vector and one of the second characteristic sensed by the first sensing vector and the second characteristic sensed by the second sensing vector; and a relationship of one of the first characteristic sensed by the first sensing vector and the first characteristic sensed by the second sensing vector and one of the third characteristic sensed by the first sensing vector and the third characteristic sensed by the second sensing vector.
 2. The method of claim 1, wherein determining the first characteristic sensed by the first sensing vector and the first characteristic sensed by the second sensing vector comprises the steps of: determining first rates of change in the cardiac signals sensed by the first sensing vector; determining second rates of change in the cardiac signals sensed by the second sensing vector.
 3. The method of claim 2: wherein determining the first rates of change comprises the steps of: determining a first rate of change threshold based, at least in part, on the cardiac signals sensed by the first sensing vector; and determining a number of the first rates of change are less than the rate of change threshold; and wherein the determining the second rates of change comprises the steps of: determining a second rate of change threshold based, at least in part, on the cardiac signal sensed by the second sensing vector; and determining number of times the second rates of change are less than the rate of change threshold; and wherein the number of times the first rates of change are less than the first rate of change threshold is the first characteristic of the cardiac signals sensed by the first sensing vector and the number of times the second rates of change are less than the second rate of change threshold is the first characteristic of the cardiac signals sensed by the second sensing vector.
 4. The method of claim 3, wherein determining each of the first rate of change threshold and the second rate of change threshold comprises: dividing the cardiac signals sensed by the first sensing vector and the second sensing vector, respectively, over the predetermined sensing window into a plurality of sub-segments; determining largest absolute rates of change for each of the plurality of sub-segments; and determining the first rate of change threshold and second rate of change threshold, respectively, in response to the largest absolute rates of change for each of the sensing vector and the second sensing vector respectively.
 5. The method of claim 4, wherein determining each of the first rate of change threshold and the second rate of change threshold further comprises: determining whether the largest absolute rates of change for each of the first sensing vector and the second sensing vector are less than a portion of a largest rate of change determined for the cardiac signals sensed by the first sensing vector and the cardiac signals sensed over the second sensing vector, respectively, over the predetermined sensing window; and setting the largest absolute rates of change for each of the first sensing vector and the second sensing vector equal to the largest rate of change for each of the first sensing vector and the second sensing vector respectively, in response to the largest absolute rates of change being less than the portion of the largest rate of change.
 6. The method of claim 1, wherein determining the second characteristic of the first sensing vector and the second characteristic of the second sensing vector comprises determining a difference between a mean frequency and a fundamental frequency of the cardiac signals sensed by the first sensing vector and the second sensing vector, respectively.
 7. The method of claim 1, wherein determining the third characteristic of the first sensing vector and the third characteristic of the second sensing vector comprises: determining a first amplitude associated with the cardiac signals sensed by the first sensing vector and a second amplitude associated with the cardiac signals sensed by the second sensing vector.
 8. The method of claim 1 wherein the first characteristic corresponds to low slope content, the second characteristic corresponds to spectral width, and the third characteristic corresponds to a normalized mean rectified amplitude.
 9. The method of claim 1, wherein the relationship of one of the first characteristic sensed by the first sensing vector and the first characteristic sensed by the second sensing vector and one of the second characteristic sensed by the first sensing vector and the second characteristic sensed by the second sensing vector is defined based, at least in part, on: a minimum one of the first characteristic of the cardiac signals sensed by the first sensing vector and the first characteristic of the cardiac signals sensed by the second sensing vector; a minimum one of the second characteristic of the cardiac signals sensed by the first sensing vector and the second characteristic of the cardiac signals sensed by the second sensing vector; and a first boundary associated with the minimum one of the first characteristic and the minimum one of the second characteristic.
 10. The method of claim 9, wherein the relationship of one of the first characteristic sensed by the first sensing vector first characteristic sensed by the second sensing vector and one of the third characteristic sensed by the first sensing vector and the third characteristic sensed by the second sensing vector is defined based, at least in part, on: a maximum one of the third characteristic of the cardiac signals sensed by the first sensing vector and the third characteristic of the cardiac signals sensed by the second sensing vector; and a second boundary associated with the minimum one of the first characteristic and the maximum one of the third characteristic.
 11. The method of claim 10, wherein delivering the therapy from the therapeutic componentry is further based, at least in part, on: both the minimum of the first characteristic being less than the first boundary and the maximum of the third characteristic being greater than the second boundary; and at least one of the minimum of the first characteristic not being less than the first boundary and the maximum of the third characteristic not being greater than the second boundary.
 12. A method of treating a cardiac event with a medical device having therapeutic componentry and a plurality of electrodes, comprising: sensing cardiac signals from the plurality of electrodes, the plurality of electrodes forming a first sensing vector and a second sensing vector; determining a first characteristic of the cardiac signals sensed by the first sensing vector over a predetermined sensing window and a first characteristic of the cardiac signals sensed by the second sensing vector over the predetermined sensing window; determining a second characteristic of the cardiac signals sensed by the first sensing vector different from the first characteristic of the first sensing vector over the predetermined sensing window and a second characteristic of the cardiac signals sensed by the second sensing vector different from the first characteristic of the second sensing vector over the predetermined sensing window; determining a third characteristic of the cardiac signals sensed by the first sensing vector different from the first characteristic of the first sensing vector and the second characteristic of the first sensing vector over the predetermined sensing window and a third characteristic of the cardiac signals sensed by the second sensing vector different from the first characteristic of the second sensing vector and the second characteristic of the second sensing vector over the predetermined sensing window; and delivering a therapy from the therapeutic componentry based, at least in part, on: a minimum one of the first characteristic of the cardiac signals sensed by the first sensing vector and the first characteristic of the cardiac signals sensed by the second sensing vector; a minimum one of the second characteristic of the cardiac signals sensed by the first sensing vector and the second characteristic of the cardiac signals sensed the second sensing vector; and a maximum one of the third characteristic of the cardiac signals sensed by the first sensing vector and the third characteristic of the cardiac signals sensed by the second sensing vector.
 13. The method of claim 12, wherein delivering a therapy from the therapeutic componentry is further based, at least in part, on: determining a first boundary corresponding to the minimum one of the first characteristic of the cardiac signals sensed by the first sensing vector and the first characteristic of the cardiac signals sensed by the second sensing vector and the minimum one of the second characteristic of the cardiac signals sensed by the first sensing vector and the second characteristic of the cardiac signals sensed the second sensing vector; determining a second boundary corresponding to the minimum one of the first characteristic of the cardiac signals sensed by the first sensing vector and the first characteristic of the cardiac signals sensed by the second sensing vector and the maximum one of the third characteristic of the cardiac signals sensed by the first sensing vector and the third characteristic of the cardiac signals sensed by the second sensing vector; delivering the therapy if the minimum one of the first characteristic is less than the first boundary and the maximum one of the third characteristic is less than the second boundary.
 14. The method of claim 13, wherein the first boundary corresponds to the sum of 0.93 and the product the minimum one of the second characteristic of the cardiac signal sensed the first sensing vector and the second characteristic of the cardiac signal sensed by the second sensing vector and −0.004, and wherein the second boundary corresponds to the sum of 8.16 and the product of 68 and the minimum one of the first characteristic of the cardiac signals sensed by the first sensing vector and the first characteristic of the cardiac signals sensed by the second sensing vector.
 15. A non-transitory computer readable medium having computer executable instructions for performing a method of treating a cardiac event with a medical device having therapeutic componentry and a plurality of electrodes comprising: sensing cardiac signals from the plurality of electrodes, the plurality of electrodes forming a first sensing vector and a second sensing vector; determining a first characteristic of the cardiac signals sensed by the first sensing vector over a predetermined sensing window and a first characteristic of the cardiac signals sensed by the second sensing vector over the predetermined sensing window; determining a second characteristic of the cardiac signals sensed by the first sensing vector different from the first characteristic of the first sensing vector over the predetermined sensing window and a second characteristic of the cardiac signals sensed by the second sensing vector different from the first characteristic of the second sensing vector over the predetermined sensing window; determining a third characteristic of the cardiac signals sensed by the first sensing vector different from the first characteristic of the first sensing vector and the second characteristic of the first sensing vector over the predetermined sensing window and a third characteristic of the cardiac signals sensed by the second sensing vector different from the first characteristic of the second sensing vector and the second characteristic of the second sensing vector over the predetermined sensing window; delivering a therapy from the therapeutic componentry based, at least in part, on: a relationship of one of the first characteristic sensed by the first sensing vector and the first characteristic sensed by the second sensing vector and one of the second characteristic sensed by the first sensing vector and the second characteristic sensed by the second sensing vector; and a relationship of one of the first characteristic sensed by the first sensing vector and the first characteristic sensed by the second sensing vector and one of the third characteristic sensed by the first sensing vector and the third characteristic sensed by the second sensing vector.
 16. A method of detecting a cardiac event in a medical device, comprising: sensing cardiac signals from a plurality of electrodes, the plurality of electrodes forming a first sensing vector and a second sensing vector; determining a first characteristic in response to the cardiac signals sensed along the first sensing vector over a predetermined sensing window and in response to the cardiac signals sensed along the second sensing vector over the predetermined sensing window; determining a second characteristic in response to the cardiac signals sensed along the first sensing vector over the predetermined sensing window and in response to the cardiac signals sensed alone he second sensing vector over the predetermined sensing window; determining a third characteristic in response to the cardiac signals sensed along the first sensing vector over the predetermined sensing window and in response to the cardiac signals sensed along the second sensing vector over the predetermined sensing window; defining a second shock zone in response to the first characteristic associated with a predetermined one of the first sensing vector and the second sensing vector and the second characteristic associated with a predetermined one of the first sensing vector and the second sensing vector; defining a first shock zone in response to the first characteristic associated with a predetermined one of the first sensing vector and the second sensing vector and the third characteristic associated with a predetermined one of the first sensing vector and the second sensing vector; and detecting the sensed cardiac signals as being associated with the event in response to the first shock zone and the second shock zone; wherein defining a first shock zone comprises: determining a minimum of the first characteristic associated with the first sensing vector and the first characteristic associated with the second sensing vector; determining a minimum of the second characteristic associated with the first sensing vector and the second characteristic associated with the second sensing vector; and defining a first boundary associated with the minimum of the first characteristic and the minimum of the second characteristic; wherein defining a first shock zone comprises: determining a minimum of the first characteristic associated with the first sensing vector and the first characteristic associated with the second sensing vector: determining a minimum of the second characteristic associated with the first sensing vector and the second characteristic associated with the second sensing vector; and defining a first boundary associated with the minimum, of the first characteristic and the minimum of the second characteristic; wherein detecting the sensed cardiac signals comprises: determining whether the minimum of the first characteristic is less than the first boundary; determining whether the maximum of the third characteristic is greater than the second boundary; identifying the both the first sensing vector and the second sensing vector as being shockable in response to both the minimum of the first characteristic being less than the first boundary and the maximum of the third characteristic being greater than the second boundary; and identifying the both the first sensing vector and second sensing vector as being not shockable in response to one of the minimum of the first characteristic not being less than the first boundary and the maximum of the third characteristic not being greater than the second boundary; and wherein the first characteristic corresponds to low slope content, the second characteristic corresponds to spectral width, and the third characteristic corresponds to a normalized mean rectified amplitude.
 17. The method of claim 16, wherein the cardiac event is ventricular tachycardia. 